Fender Stage 112 repair - how to tackle burnt PCB

[ralphonz]

Hi guys,

This is my first post and also my first time repairing an amp. Please go easy

I'm taking a look at my band-mates fender 112. It has two burnt resistors (R75 & R76) which have almost burnt a hole on the top and on the bottom the pads and traces have lifted.




Can anyone please advise me on the best way to treat this and get the amp working again.

I alredy tried replacing the resistors using the bits of trace and pad still left there, and although the amp was better for a short while it still didn't work properly - the problem being that the volume would suddenly drop after 30 seconds or so of playing and become really distorted, then intermittently would go back up again....

If anyone can tell me why the resistors burnt in the first place and if there is anything else I should check (though all caps look fine) then I'm all ears! I'm here to learn

[Mick Bailey]

Those resistors along with C49 form a 'Zobel network'. If you look at the schematic, that capacitor hangs off the +ve speaker connection and this is connected to the paralleled resistors that then connect to ground. Maybe C49 is shorted/shorting, or you have an issue that's putting excessive high-frequency across the network. Make sure the connections to L1 and R82 are good. In fact, while you're at it inspect the board around those components for bad solder joints. With components like the capacitor if I suspect them I replace them and don't bother testing.

To repair your board you need to cut away the lifted section cleanly with a scalpel. I would also clean away any carbon. It's convenient that the resistors are paralleled because you can twist the leads together on the copper side of the board to give a secure mounting. Solder the twist and clip it neatly, then attach insulated wires to where the tracks originally ran to the next solder pad. I secure the leads with a wire-tack adhesive, but a dab of silicone or hot-melt can be used sparingly. Just enough to secure the leads against the PCB

[bob p]

wire tack adhesive?

[ralphonz]

Those resistors along with C49 form a 'Zobel network'. If you look at the schematic, that capacitor hangs off the +ve speaker connection and this is connected to the paralleled resistors that then connect to ground. Maybe C49 is shorted/shorting, or you have an issue that's putting excessive high-frequency across the network. Make sure the connections to L1 and R82 are good. In fact, while you're at it inspect the board around those components for bad solder joints. With components like the capacitor if I suspect them I replace them and don't bother testing.

To repair your board you need to cut away the lifted section cleanly with a scalpel. I would also clean away any carbon. It's convenient that the resistors are paralleled because you can twist the leads together on the copper side of the board to give a secure mounting. Solder the twist and clip it neatly, then attach insulated wires to where the tracks originally ran to the next solder pad. I secure the leads with a wire-tack adhesive, but a dab of silicone or hot-melt can be used sparingly. Just enough to secure the leads against the PCB

Thanks Mick,

very helpful info. I had looked at the schematic and suspected something funky was going on with that little circuit What exactly do you mean by "clean away any carbon"? The black charred stuff? Time to read up on "Zobel Network" and order the bits and bobs i need to fix it. Thanks again, I think I'm going to enjoy this forum.

[ralphonz]

wire tack adhesive?

This sort of thing?
https://uk.rs-online.com/web/p/cyano...sives/1987144/
Seems pricey, is it worth investing in some?

[52 Bill]

Welcome to the place.

Often when I find those resistors burned or overheated like that, I find that some of the small value feedback loop caps in the preamp section have broken off or have come unsoldered. Caps like C13 or C19.

You should grind away the burnt, carbonized part of the pc board as much as possible without damaging the underside copper traces. Carbon is conductive and while the amp you are working on will probably not be effected by it, it is good practice to clean it out when it occurs.

[bob p]

This sort of thing?
https://uk.rs-online.com/web/p/cyano...sives/1987144/
Seems pricey, is it worth investing in some?

LMAO. Ethyl cyanoacrylate is Crazy Glue.

[Mick Bailey]

The stuff I use is rubberized and specifically used to secure wiring in aircraft assemblies. Super-expensive but I get it cheap due to it being close to its expiry date. It still lasts for ages and works perfectly. Just like you can still eat a can of peaches three years out of date.

I've seen those resistors burn when there's high-frequency oscillation. Oscillation can produce a lot of energy that causes distortion and saps the amp of power, though you can't hear it. Above a certain frequency C49 will act like a straight piece of wire and then there are just the resistors in the way to cook. It can also damage your speaker. Boards in amps can suffer solder cracks and I use an illuminated bench magnifier to inspect the solder joints. Your eyesight may be better than mine.

[bob p]

I'd like to know more about the rubberized stuff. I have an amp with a burned ground trace that needs to be fixed and I've been putting that off...

[ralphonz]

Those resistors along with C49 form a 'Zobel network'. If you look at the schematic, that capacitor hangs off the +ve speaker connection and this is connected to the paralleled resistors that then connect to ground. Maybe C49 is shorted/shorting, or you have an issue that's putting excessive high-frequency across the network. Make sure the connections to L1 and R82 are good. In fact, while you're at it inspect the board around those components for bad solder joints. With components like the capacitor if I suspect them I replace them and don't bother testing.

I've tested C49 which looks to be within tolerance - I read 107nf. All the joints for C49, L1 and R82 look good so I'm assuming the problem was in the bad pad/track from R76 to C49 (or R76 to ground?). In your experience would there be anything else to check before I put it back together to test (it's not the easiest board to get out of the chassis!)?

Thanks for the responses everyone.

[Mick Bailey]

Whilst the value reads OK, this doesn't tell you if a cap shorts out in operation. Cap value is not an indication of whether its good or not, though no reading or zero is clearly a problem. Whatever caused those resistors to overheat in the first place was most likely not a bad pad or bad track. The resistors cooked because of excessive voltage (resulting in their wattage rating being exceeded). So even if you replace the resistors and wire them in you're no further forward - you still have the original fault. What level of equipment do you have to hand - DMM/scope/variac? At this stage I would repair the original damage (and throw in a new C49 just to be sure) and be scoping around the output looking for oscillation. In the absence of a scope a decent DMM will detect oscillation. The schematic has useful test voltages and outlines the test conditions in the notes section.

Again, did you thoroughly check the board for cracked joints? A consistent problem with amps is circular cracks around component legs. Sometimes the leg is loose.

[Enzo]

In my experience, there is only one thing that bur s up those resistors - HF oscillation in the amp. If teh cap shorted, they would be across teh output, but I don't recall ever finding a shorted cap there.

[ralphonz]

Thanks Mick,

Makes perfect sense, yes the original fault that caused the resistors to burn must still be present.

I don't have a scope or variac but I do have a DMM. I've looked all over the board and all the joints look shiny and well-shaped, nothing seems loose or cracked. To be honest I don't know where to start looking for HF oscillation with a DMM, this is my first amp repair.

When I had a guitar plugged into the amp prior, the power amp input worked fine, only input 1 and 2 exhibited the behaviour I described in my first post - I should have mentioned that before.

[Mick Bailey]

Ah. So that means the power amp is OK and probably what we're looking at is a fault that lies in the preamp. I'm thinking that 52 Bill maybe had the answer in post #6 and those caps should be examined in more detail. So that would be C13, C19, C30, C34, C37. These are in the feedback loops of the opamps and serve both to stabilize the circuits to prevent oscillation and also to prevent broadcast radio from being picked up and amplified.

A good DMM should be able to 'read' oscillation - certainly my Fluke does this when set to AC volts mode (mine autoranges but manually you could begin with 200mV).

You see on the schematic where those caps are located? Pins 1 or 7 respectively are the opamp outputs. If you insert a plug into the 'power amp in' socket you'll disconnect the power amp to prevent further damage. Ground the negative lead of your DMM and check with the +ve lead to see if there's any AC voltage on the outputs of those opamps. There's usually a little due to power supply ripple and noise, but should be no more than a few mV at most. Sometimes an oscillating opamp will get much hotter than the rest. Turning up the controls will give you the best chance of spotting anything wrong.

The only caveat with this is the multimeter itself (or scope probe) can sometimes stop any oscillation.

[Enzo]

The zobel network is there to protect the PA from itself. If the PA turns into a power oscillator is can shake itself to death. Just sending it a steady HF signal from the preamp won't hurt the PA. The PA can break into oscillation at 50kHz even, and my meter won't read that.